A cognitive approach to grammar...
... captures the idea that the act of deploying a symbolic unit in any given usage event involves both semantic space (meaning) and phonological space (form).
Source: Langacker 1987
The grammar box represents the conventionalized knowledge of language in the mind of a speaker. The usage box on the other hand represents the usage event or utterance. A usage event consists of speech sounds (vocalisation) and their corresponding interpretations (conceptualization).
The horizontal arrows represent coding links or correspondences between the conventionalizes units of knowledge in the mind and the vocal or conceptual systems they interact with in instances of situated language use, i.e. the semantic pole of a linguistic expression corresponds to a concept, the phonological pole to the string of sounds that realize it. The vertical arrows represent symbolic links which unite sound and meaning.
There is no distinction between competence and performance: everything is interrelated!